1. Field of the Invention
This invention relates a process for preparing a coating type magnetic recording medium suitable for high density recording.
2. Related Background Art
In recent years, as the recording medium for computer, word processor, electronic still camera, etc., a disc-shaped flexible recording medium has been used. In such disc-shaped flexible magnetic recording medium, since reading and writing of recording are effected by sliding of a magnetic head in the circumferential direction (concentric shape) of the magnetic suface, if the magnetic particles are oriented in one direction as in magnetic tape, difference occurs between the reproduction output in the oriented direction and the reproduction output in the direction perpendicular to the oriented direction, which appears as the output fluctuation (modulation). Accordingly, in the disc-shaped magnetic recording medium used for a magnetic disc device, the magnetic particles must be randomly oriented (namely non-oriented) within the magnetic recording layer plane.
However, in preparing a coating type flexible magnetic recording medium, since a support carrying a magnetic coating material is passed between two rotatory rollers having a gap slightly broader than the thickness of the support formed therebetween, the magnetic particles are oriented in the running direction of the support. This is called the mechanical orientation. The magnetic recording medium subjected to the mechanical orientation will generate the modulation as mentioned above. Generally speaking, magnetic particles with greater shape anisotropy, namely with greater axis ratio are susceptible to mechanical orientation.
For inhibiting such modulation, it has been considered to employ a method to make the magnetic particles non-oriented by giving a magnetic field to the magnetic recording layer before the magnetic recording layer on the support is dried. For example, there have been known various methods as follows:
(a) the method in which the magnetic field is minutely varied with fine magnets arranged as directed toward various directions (Japanese Patent Publications Nos. 60-11370, 59-23010, Japanese Patent Laid-open Applications Nos. 59-18838 and 61-160825);
(b) the method in which several magnets are arranged at the upper portion and the lower portion with the support sandwiched therebetween (Japanese Patent Publication No. 45-21547, Japanese Patent Laid-open Application No. 54-149607);
(c) the method in which a magnetic field is applied in the width direction of the support (Japanese Patent Laid-open Applications Nos. 58-215731, 59-203243);
(d) the method in which a rotatory magnet which rotates with the rotation surface parallel to the support surface (Japanese Patent Laid-open Application No. 57-186232), etc.
However, in all of the above prior arts, it has been difficult to randomize sufficiently the ferromagnetic particles with great coercive force as used in a disc-shaped recording medium suitable for high density recording. Thus, even with the magnetic line of force applied for randomization, because of having itself inherent directionality, no sufficient randomization could be accomplished ((a), (b), (c)).
On the other hand, in the method (d) wherein the magnet is rotated so that the rotation surface may become in parallel to the support surface in order to remove inherent directionality of the magnetic line of force, there has been involved the problem that the surface roughness of the magnetic recording layer is worsened.